The organic light-emitting device was first reported on its high-luminance emission by C. W. Tang et al of Kodak in 1987 (see, Appl. Phys. Lett., Vol. 51, page 913 (1987)). Since then, an abrupt progress has been proceeding in the development of materials and improvement of device structures and in recent years, the organic light-emitting device is actually used in a display for car audios or cellular phones. In order to more expand the use of this organic electroluminescent (EL) device, development of materials for improving the performance of the device in terms of emitting efficiency, luminescent color or durability, or development of full color displays are being aggressively made at present. In order to further improve the performance of the organic light-emitting devices, it is necessary to establish a device structure suitable for the material properties and a method for producing devices as well as to develop light-emitting materials. Approaches to improving properties of devices differ depending on whether the light-emitting material is a low-molecular or polymer substance. The background art is described starting from a low-molecular substance.
The light-emitting material used for the organic electroluminescent device proposed by C. W. Tang et al was a low-molecular compound. It has not changed then and now that a vacuum deposition method has been widely used to form organic electroluminescent devices using a low-molecular compound as the light-emitting material. In the organic electroluminescent device proposed by C. W. Tang et al, two kinds of low-molecular compounds are sequentially deposited on an anode substrate by vapor deposition, and a cathode is further deposited thereon to form a device having a two-layer structure. One organic layer is selected to inject and transport holes (a hole transporting compound) and the other organic layer is selected to inject and transport electrons (an electron transporting compound). The interface between the two layers provides a site for recombination of electrons with holes and a resulting light emission. (See U.S. Pat. No. 4,356,429 and other numbers of patent publications for specific examples.)
The above-mentioned organic electroluminescent device having a two-layer structure in the early development has greatly developed, and numerous examples have been given in which the improvement of the device performance is attained by adopting a multilayer structure having three or more layers.
Thus, in order to convert the electric energy into light energy with minimum loss, it has been attempted to improve the device performance by combining organic compounds having different carrier transporting properties and further by layer-by-layer depositing the compounds in a best suited order. The improvement of the material necessary for attaining high performance has been made not only on a hole transporting or electron transporting material but also on a light emitting material. As a result, by using a phosphorescent material in place of a fluorescent material, external quantum efficiency exceeding 5% which is said to be the maximum with a fluorescent material has been attained (Proceedings of the 11th International Workshop on Inorganic and Organic Electroluminescence (EL2002), pages 283-286, 2002).
The factor attributing to improvement of the device performance is not only the light-emitting material constituting the device but the constitution of an anode and a cathode to apply voltage to the light-emitting material greatly influences the performance as well.
It is well known now that the operating voltage of an organic electroluminescent (EL) device may notably lower by using a low work function cathode and a high work function anode. A preferable cathode is, as described in U.S. Pat. No. 4,885,211 by Tang et al and U.S. Pat. No. 5,059,862 by Van Slyke et al, constructed by combining a metal having a work function lower than 4.0 eV and a metal having a work function higher than 4.0 eV. U.S. Pat. No. 5,677,572 by Hung et al describes using LiF—Al double layers in order to improve electron injection of an organic electroluminescent (EL) device.
In an organic electroluminescent device, an anode is usually formed of indium tin oxide (ITO) on account of its transparency, high electrical conductivity and high work function. However, an organic electroluminescent device produced by forming a film of a hole-transporting compound directly on the surface of bare ITO generally has an insufficient current-voltage property and a lower operating stability. It is mainly attributed to a high injection barrier for the holes, dielectric breakdown by an electric field caused by projections on the surface of ITO. One way to avoid the problems is a method to introduce an intermediate layer between ITO and the hole-transporting compound. For example, Van Slyke et al illustrates that when copper phthalocyanine (CuPc) is pre-attached onto the surface of ITO, the organic device produced therefrom has improved stability (Organic electroluminescent devices with improved stability, S. A. Van Slyke, C. H. Chen and C. W. Tang, Applied Physics Letters, Vol. 69, 2160, 1996). However, when a CuPc layer is inserted between ITO and the hole-transporting compound, the hole injection barrier present on the interface between CuPc and the hole-transporting layer is still high, which leads to increasing the driving voltage. Starting from CuPc, extensive studies have been made on development of materials in order to alleviate the injection barrier for the holes. In any case, making use of the characteristics of low-molecular compounds, it has been attempted to improve the device performance by layer-by-layer deposition of a most suitable compound on the surface of an anode using vacuum deposition.
As mentioned above, though vacuum deposition is a method widely used for forming a film of a low-molecular light-emitting material, it is disadvantageous in that a vacuum apparatus is required. Moreover, the larger the area of the organic thin film to be formed is, the more difficult it is to form the organic thin film with a uniform thickness and to form a high-definition patterning. Thus, the method is not necessarily suitable for mass production of large area panels. Also, the method is accompanied by difficulty in forming a multilayer structure with an appropriate film thickness.
As a method to solve the problems, a spin coating method, ink-jet method and printing method, which are considered to be suitable for area enlargement and mass-production of organic light-emitting devices, have been developed as a film-forming method. In these methods, coating of a light-emitting layer is completed when a light-emitting polymer material dissolved in an organic solvent is coated to form a film on an anode. Vapor deposition is performed only when forming a cathode. Even in the case of an organic light-emitting device using a light-emitting polymer material, improvement of materials is indispensable for attaining high performance devices just as in the case of a device using a low-molecular light-emitting material. And an approach to the improvement differs from that in the case of a device having a multilayer structure of low-molecular materials.
In a typical structure of the organic light-emitting device of the present invention, an anode (transparent), an intermediate layer, a light-emitting layer and a cathode are formed in this order on a transparent substrate. Here, the intermediate layer may be called an anode buffer layer and is inserted for the purposes of preventing an electric short-circuit by smoothing the surface of the anode and alleviating the barrier against the hole injection from the light-emitting layer to the anode. A role required to the anode buffer layer is almost the same as that of CuPc proposed in producing a low-molecular organic light-emitting device. A polymer organic light-emitting device is produced by coating a polymer dissolved in an organic solvent, while a low-molecular organic light-emitting device is produced by depositing an upper layer onto a lower layer subsequently by a dry process. Accordingly, the properties required for the intermediate layer in a polymer organic light-emitting device differ slightly from those in the case of a low-molecular organic light-emitting device. That is, the following two points are critical during the step of coating an upper layer onto the intermediate layer: components of the intermediate layer material must not be dissolved in a solvent used for coating the upper layer on the intermediate layer, and if the components may not be dissolved in a solvent, they should not be peeled off or diffused by a physical impact at the time of coating. Furthermore, when the drying step is completed after coating, the interface between the upper and intermediate layers should have sufficient adherence. On that condition, the intermediate layer is required to have electric properties (capable of smoothing the anode surface, alleviating the barrier for hole injection, etc.) as revealed with a low-molecular organic electroluminescent device, and physical and chemical properties (should not undergo changes such as crystallization and dispersion into multiple layers accompanied by energization and time passage).
An aqueous solution of a mixture of polyethylene dioxythiophene (PEDOT) and polystyrene sulfonate (PSS) is generally used for the intermediate layer. With respect to this intermediate layer, a problem has been pointed out that polystyrene sulfonate contained as an external dopant infiltrates the upper light-emitting layer and thereby deteriorates it.
With respect to such a problem caused by mobile counter ions contained in the anode buffer layer, though Published Japanese Translation of PCT international publication No. 2003-509816 (WO2001/018888) discloses a method of using an anode buffer layer of self-doping polyaniline, it has not yet succeeded in exhibiting sufficient properties of the layer.
As technologies relating to the present invention, Japanese Laid-Open Patent Publication No. 2000-311869 discloses a surface modifying method of an organic electroluminescent device by radio frequency (RF) plasma treatment. However, the plasma application differs from that in the present invention in which a new layer is formed on the surface of an anode. Japanese Laid-Open Patent Publication No. 2000-150171 (EP 1026757 A2) discloses an organic electroluminescent device wherein a polymer thin film having a thickness of 0.2 to 3 nm, preferably 0.4 to 1 nm, is formed on the anode by radio frequency plasma polymerization, and a light-emitting multilayer structure is provided thereon. However, the publication does not describe a single light-emitting layer containing a polymer as a light-emitting material.